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Blue hole is a term for water-filled sinkholes with the entrance below the water level. They can be formed in different karst processes, for example, by the rainwater soaking through fractures of limestone bedrock onto the watertable. Sea level here has changed: for example, during the glacial age during the Pleistocene epoch (ice age), some 15,000 years ago, sea level was considerably lower. The maximum depth of most other known blue holes and sinkholes is 110 metres (360 ft), which makes the 202 metres (663 ft) depth of Dean's Blue Hole quite exceptional.
Dean's Blue Hole is roughly circular at the surface, with a diameter ranging from 25 to 35 metres (82–115 ft). After descending 20 metres (66 ft), the hole widens considerably into a cavern with a diameter of 100 metres (330 ft).
There are several freshwater water-filled sinkholes on land that are deeper than Dean's Blue Hole. These include the 270 m (890 ft) Boesmansgat in South Africa, Mexico 's Zacatón at 335 metres (1,099 ft) and the 392 metres (1,286 ft) Pozzo del Merro in Italy.